Spinal cord stimulation is a technique which uses an implanted electrode array to control chronic pain. The electrode array is typically implanted in a fixed position within the epidural space near the spinal cord. A signal generator delivers current pulses to the spinal cord via the implanted electrode array. The current pulses induce parasthesiae which help block the perception of pain.
In FIG. 1, spinal column 1 is shown to have a number of vertebrae, categorized into four sections or types: lumbar vertebrae 2, thoracic vertebrae 3, cervical vertebrae 4 and sacral vertebrae 5. Cervical vertebrae 4 include the 1st cervical vertebra (C1) through the 7th cervical vertebra (C7). Just below the 7th cervical vertebra is the first of twelve thoracic vertebrae 3 including the 1st thoracic vertebra (T1) through the 12th thoracic vertebra (T12). Just below the 12th thoracic vertebrae 3, are five lumbar vertebrae 2 including the 1st lumbar vertebra (L1) through the 5th lumbar vertebra (L5), the 5th lumbar vertebra being attached to sacral vertebrae 5 (S1 to S5), sacral vertebrae 5 being naturally fused together in the adult.
In FIG. 2, representative thoracic vertebra 10 is shown to have a number of notable features which are in general shared with lumbar vertebrae 2 and cervical vertebrae 4. The thick oval segment of bone forming the anterior aspect of vertebra 10 is vertebral body 12. Vertebral body 12 is attached to bony vertebral arch 13 through which spinal nerves 11 run. Vertebral arch 13, forming the posterior of vertebra 10, is comprised of two pedicles 14, which are short stout processes that extend from the sides of vertebral body 12 and bilateral laminae 15. The broad flat plates that project from pedicles 14 join in a triangle to form a hollow archway, spinal canal 16. Spinous process 17 protrudes from the junction of bilateral laminae 15. Transverse processes 18 project from the junction of pedicles 14 and bilateral laminae 15. The structures of the vertebral arch protect spinal cord 20 and spinal nerves 11 that run through the spinal canal.
Surrounding spinal cord 20 is dura 21 that contains cerebrospinal fluid (CSF) 22. Epidural space 24 is the outermost part of the spinal canal. It is the space within the spinal canal formed by the surrounding vertebrae lying outside the dura.
Referring to FIGS. 1, 2 and 3, the placement of an electrode array for spinal cord stimulation according to the prior art is shown. Electrode array 30 is positioned in epidural space 24 between dura 21 and the walls of spinal canal 16 towards the dorsal aspect of the spinal canal nearest bilateral laminae 15 and spinous process 17.
FIG. 4 shows a prior art electrode array 30 including a set of electrode contacts 35 sealed into elastomeric housing 36. Electrode array 30 has a set of electrode leads 31 which are connected to electrical pulse generator 32 and controller 33. The electrical pulse generator may be outside of the body or it may be implanted subcutaneously. Each electrode contact has a separate electrical conductor in the set of electrode leads 31 so that the current to each contact may be independently conducted and controlled.
The anatomical distribution of parasthesiae is dependent upon the spatial relationship between a stimulating electric field generated by the electrode array and the neuronal pathways within the spinal cord. The distribution may be changed by altering the current across one or more electrodes of the electrode array. Changing anode and cathode configurations of the electrode array also alters the distribution and hence, the anatomical pattern of the induced parasthesiae.
Proper intensity of the current pulses is important. Excessive current produces an uncomfortable sensation. Insufficient current produces inadequate pain relief. Body motion, particularly bending and twisting, causes undesired and uncomfortable changes in stimulation due to motion of the spinal cord relative to the implanted electrode array.
There are methods and systems for controlling implanted devices within the human body. For example, Ecker et al, in U.S. Patent Application No. 2010/0105997, discloses an implantable medical device that includes a controller and a plurality of sensor modules. A sensor includes at least one light source that emits light at a particular wavelength, which scatters through blood-perfused tissue a detector senses the light reflected by blood mass of a patient.
U.S. Pat. No. 7,684,869 to Bradley, et al. discloses a system using an interelectrode impedance to determine the relative orientation of a lead other leads in the spinal column. Bradley et al. further disclose that interelectrode impedance may be used to adjust stimulation energy.
U.S. Patent Application No. 2009/0118787 to Moffitt, et al. discloses electrical energy conveyed between electrodes to create a stimulation region. Physiological information from the patient is acquired and analyzed to locate a locus of the stimulation region. The stimulation region is electronically displaced.
Deficiencies exist in the prior art related to accuracy of spinal cord stimulation in relieving pain under changing circumstances. The deficiencies are most pronounced while the patient is moving. The prior art does not provide a satisfactory way to automatically adjust spinal cord stimulation to compensate for motion between the electrodes and the spinal cord to maintain a constant level of pain relief during patient motion.